Work will be continued on the effects of postnatal corticosterone treatment on the developing brains of mice and rats, with the objective of elucidating the nature of the changes in behavior and in brain composition associated with the growth impairment produced by the steroid. Animals will be treated at different ages, and the steroid effect compared with that of growth reduction to the same body weight by nutritional restriction. DNA synthesis and possible cell lysis will be examined by giving tritiated thymidine at various ages, before and (in different animals) during steroid treatment, followed by counting of the DNA fraction of various brain regions. Also, after autoradiography, counts of isotopically labeled cells will be made in 6 micron serial sections, with the objective of identifying the cell types that are absent after the growth suppression, and thus to account cytologically for the observed reduction in brain DNA. Experimentals will be matched with litter-mate controls. The histology will be correlated with studies of the ganglioside and cerebroside content, and with the S-100 protein, to see to what extent these presumptive markers of neuropile, myelin, and glia might be altered by the experimental treatments. Possible effects of the experimental treatments on tryptophan hydroxylase and phenyl-ethanolamine-N-methyl transferase will be explored. Study of the biochemical and histological maturation of the normal monkey brain will be continued, with the objective of providing a specific basis for estimating the age periods at which a primate brain might be expected to be vulnerable to the factors that can modify rodent brain development.